Method for regulating a predetermined modifiable brake pressure

ABSTRACT

The present invention relates to a method for regulating a predetermined modifiable brake pressure in the wheel brakes of a brake system, wherein input quantities determining the brake pressure in the individual wheel brakes are evaluated and correcting variables of hydraulic valves are defined in a control and/or data processing system. To obviate the need for additional pressure sensors which register the pressure in the wheel brakes, it is arranged for by the invention that a characteristic curve is stored in the control or data processing system, said curve correlating the valve characteristics of the hydraulic valve with a pressure difference of the hydraulic valve, and the correcting variable of the hydraulic valve is defined according to the characteristic curve.

The present invention relates to a method for regulating a predeterminedmodifiable brake pressure in the wheel brakes of a brake system, whereininput quantities determining the brake pressure in the individual wheelbrakes are evaluated and correcting variables of hydraulic valves aredefined in a control and/or data processing system.

Vacuum brake boosters require a vacuum supplied by the engine forboosting the pedal force to be generated by the driver. Depending on theengine, even relatively low pedal forces allow reaching a conditionwhere further increase of the force applied to the actuating unit ispossible only by an increase in the pedal force because the vacuum brakebooster has reached the maximum possible boosting force. This conditionis referred to as the point of maximum boosting of the booster. Brakingoperations that take place in excess of the point of maximum boosting ofthe vacuum brake booster place high demands on the driver in terms ofthe pedal force to be generated. For this reason, brake systems (OHB-V)are employed at an increasing rate supporting the driver by means of anactive hydraulic brake pressure build-up. Brake systems of this typegenerally include a motor-and-pump assembly and a hydraulic unit beingcontrolled by an electronic control such as ESP, ABS, TCS, and similarsystems, to comply with the desired brake force boosting.

When pressure is built up in the wheel brakes by means of hydraulicboosting beyond the point of maximum boosting of the vacuum brakebooster, the (analogized) separating valves are closed, and thehydraulic pump delivers brake fluid from the tandem master cylinder(TMC) into the wheel brakes. In the pressure reduction phase, it isrequired to discharge the additionally generated pressure into thetandem master cylinder by way of the separating valves in a mannercomfortable with respect to pedal feeling and braking effect. Thisaction usually requires two additional pressure sensors at the wheelbrakes, one per brake circuit, in order to determine the wheel pressure.The result is a higher requirement of components causing an increase inthe system's costs.

In view of the above, an object of the invention is to provide a methodfor regulating a predetermined variable brake pressure in the wheelbrakes of a brake system, wherein brake pressures can be adjustedaccording to input quantities without any knowledge about a measuredwheel pressure.

According to the invention, this object is achieved in that acharacteristic curve is stored in the control or data processing system,said curve correlating the valve characteristics or the valve current ofthe hydraulic valve with the pressure difference of the hydraulic valve,and the correcting variable of the hydraulic valve is defined accordingto the characteristic curve. Advantageously, hydraulic boosting of thebrake pressure introduced into the brake system is brought about bymeans of the predetermined variable brake pressure.

It is favorable that the correcting variable is an electric valvecurrent by means of which the hydraulic valve is controlled in analogmanner.

With the knowledge of the valve characteristic curve of a separatingvalve, the tandem maser cylinder pressure and the nominal pressure forthe wheel brakes, it is possible to define and adjust a valve current insuch a fashion that a defined wheel pressure is achieved, and fallingshort of said pressure will not occur without the need for additionalpressure sensors measuring the wheel pressures.

Advantageously, the correcting variable is determined from the pressuredifference between the nominal pressure for the wheel brakes and thepressure of an actuating device (pressure difference of the hydraulicvalve). The pressure of the actuating device (TMC=tandem mastercylinder) is determined by way of a pressure sensor provided in a brakecircuit in an ESP control, and the nominal pressure is determined in aknown model.

To reduce leakages, on the one hand, and allow limiting the maximumpressure in response to the situation, on the other hand, at least twostrategies are provided for regulating the brake pressure. While thevalve current follows the course of the characteristic curve accordingto the first strategy, the second strategy arranges for jump functionsat the maximum actuating current or the actuating current represented bythe characteristic curve. The maximum actuating current is limitedcorresponding to a function according to I_(max)=f (P_(max), P_(TMC),P_(PLV))=min(P_(max), P_(TMC), P_(PLV)) (PLV=pressure limiting valve).

A device of the invention for implementing the method as claimed in anyone of claims 1 to 5 is designed so that the brake pressure generated byactuation of a pump is controlled in such a way that a desired hydraulicboosting of the introduced brake pressure is achieved in each brakecircuit.

An embodiment of the invention is illustrated in the accompanyingdrawings and will be described in detail in the following.

In the drawings,

FIG. 1 is a view showing a brake system with two brake circuits.

FIG. 2 is a schematic view of the signal variation of the actuatingcurrent of a hydraulic valve and the pressure according to the method ofthe invention.

FIG. 3 is a characteristic curve of the invention.

FIG. 4 is a schematic view of the signal variation of the actuatingcurrent with an additional energization.

The one brake circuit of a brake system for motor vehicles with twobrake circuits as shown in FIG. 1 consists of an actuating unit 1, e.g.a brake cylinder, and a brake force booster 2 actuated by a brake pedal3. Arranged at the actuating unit 1 is a supply tank 4 that containspressure fluid volume and is connected to the working chamber of theactuating unit in the brake's release position. The one brake circuitshown includes a brake line 5 connected to a working chamber of theactuating unit 1 and containing a separating valve 6 that provides inits inactive position an open passage for the brake line 5. Theseparating valve 6 is typically actuated electromagnetically.

The brake line 5 branches into two brake lines 8, 9 respectively leadingto a wheel brake 10, 11. Brake lines 8, 9 respectively contain anelectromagnetically operable inlet valve 12, 19. Said valves are open intheir inactive position and can be operated to adopt their closedposition by energizing of the actuating magnet. Connected in parallel toeach inlet valve 12, 19 is a non-return valve 13 that opens in thedirection of the brake cylinder 1. A so-called return circuit thatcomprises return lines 15, 32, 33 with a pump 16 is connected inparallel to wheel brake circuits 26, 27. By way of each one outlet valve14, 17 and through return lines 32, 33, the wheel brakes 10, 11 areconnected to the return line 15 and, hence, to the suction side of thepump 16 whose pressure side is connected to the brake pressure line 8 ina mouth E between the separating valve 6 and the inlet valves 12, 19.

Pump 16 is designed as a reciprocating piston pump with a pressure valve(not shown) and a suction valve. A low-pressure accumulator 20, composedof a housing 21 with a spring 22 and a piston 23, is arranged at thesuction side of the pump 16.

A biased non-return valve 34 opening towards the pump is interposed intothe connection between the low-pressure accumulator 20 and the pump 16.

The suction side of the pump 16 is further connected to a low-pressuredamper 18 by way of a suction line 30 and to the brake cylinder 1 by wayof a change-over valve 31. In addition to the hydraulic unit 43, thebrake force transmission circuit also includes a device 28 forcontrolling the brake system. Said device basically is an ESP controlunit 45, associated with which is a model 41 for determining the nominalbrake pressure and an accumulator 42 for storing the valvecharacteristic curve that describes the valve current and thecorresponding difference in pressure at which the separating valve 6opens. The pressure sensor 40 detecting the pressure of the actuatingunit 1 is arranged in the brake line 5 between the brake cylinder 1 andthe change-over valve 31 or the separating valve 6, respectively. Wheelspeed sensors associated with the wheels are designated by referencenumerals 50, 51. Input quantities that are sent to the ESP control unit45, such as the signals of the rotational speed sensors, at least oneyaw sensor, of one acceleration sensor or pressure sensor 40 areexemplarily designated by reference numerals 55 to 57.

The brake system operates as follows:

When braking, for example, the driver increases the brake pressure inthe hydraulic unit 43 by way of the pedal 3 and the actuating unit 1with the vacuum brake booster 2, without the vehicle deceleratingcorresponding to the pedal force. When braking by pedal depression, thedevice 28 evaluates the pressure of the actuating unit 1 determined bythe pressure sensor 40 or the brake pressure introduced into the brakeline 5. When the pressure reaches a limit value which describes themaximum boosting pressure of the actuating unit or the point of maximumboosting of the vacuum brake booster 2, the transition from thepneumatic brake force boosting by way of the vacuum brake booster 2 tothe active brake force boosting by means of the pump 16, is carried outin particular according to the relationP_(nominal)=P_(point of max. boosting)+factorK×(P_(TMC)−P_(point of max. boosting)), with P_(nominal)=nominalpressure, P_(point of max. boosting)=pressure at the point of maximumboosting of the brake booster, P_(TMC)=pressure of the actuating unit.To this end, the (analogized) separating valves 6 are closed in thepressure build-up with the change-over valve 31 open, and the hydraulicpump 16 delivers brake fluid out of the brake cylinder 1, e.g. a tandemmaster cylinder (TMC) into the wheel brakes 10, 11. The inlet valves 12or 19, respectively, are open, while the outlet valves 14 or 17,respectively, are closed. In the pressure reduction phase, theadditionally produced pressure is discharged through the analogizedseparating valves 6 into the brake cylinder 1 in a manner that iscomfortable with respect to pedal feeling and braking effect.

FIG. 2 shows a schematic view of the signal variation of the actuatingcurrent I₀, I of the separating valve 6 and the differential pressureP₀, P_(TMC), P_(nominal).

This actuating current is determined from the difference between thenominal pressure P_(nominal) for the wheel brakes 10, 11 and themeasured pressure of the brake cylinder (TMC) 1. If a higher pressureprevails in the brake cylinders of the wheel brakes 10, 11, theseparating valve 6 will open, and fluid propagates back into the brakecylinder 1 until the desired pressure has been adjusted in the hydraulicunit 43. When pressure balance prevails between the lines 5 and 8, 9 orbetween the inlet and outlet of the separating valve 6, the separatingvalve 6 will close and the pressure is maintained. To speed up theadjustment operation, the nominal pressure gradient of the brakepressure P_(nominal) can be evaluated and, hence, the valve current Imodified accordingly. This achieves a greater volume flow through theseparating valve 6. This means, the reaction to quick pressure increaserequirements is that the separating valve 6 is opened to a largerextent.

To actuate the separating valve 6, the valve characteristic curve 70(FIG. 3) is stored in non-volatile memory 42 and describes the valvecurrent I₀, I, I_(max) and the corresponding pressure difference P₀,P_(nominal), P_(TMC), at which the separating valve 6 opens. Saidcharacteristic curve 70 can be determined by a measurement of the valvesor by a calibration at the end of the assembly line. The calibration atthe end of the assembly line represents a favorable variant because thecomplete chain of influencing factors (drivers, coils, valve, etc.) isincluded in the stored characteristic curve at this point. Saidcharacteristic curve permits controlling the pressure produced byactuation of the pump 16 in such a fashion that the desired hydraulicboosting can be adjusted to an appropriately greater accuracy withoutthe driver being able to notice the difference compared to thecontrolled system (with wheel pressure sensors).

Principally, braking operations can be subdivided into the sectionspressure increase, maintaining the pressure constant, and pressurereduction. In each of the braking cycles, two strategies are generallyprovided to regulate the brake pressure. According to the firststrategy, the brake pressure follows the course of the characteristiccurve 70, while the second strategy arranges for jump functions at themaximum actuating current or the actuating current represented by thecharacteristic curve.

I. Pressure Increase

Strategy A: The valve current I used to adjust the separating valve 6 ismodified corresponding to the characteristic curve I=I (P_(nominal))shown in FIG. 3.

Advantage: The pressure adjusted in wheel brakes 10, 11 is limited inits magnitude. When the pump 16 builds up too much pressure in at leastone brake circuit, the portion in excess of the nominal pressureP_(nominal) can flow off through the separating valve 6. This case canoccur especially at the rear axle in brake systems with black-and-whilecircuit allotment.

Possibility B: Jump to the making current for differential pressure zero(I₀) and, subsequently, a quick ramp 1 to the maximum current I_(max),according to the relation I_(max)=f(P_(max), P_(TMC), P_(PLV))=min(P_(max)−P_(TMC), P_(PLV)) with P_(PLV)=pressure limiting valve pressureshut off, P_(max)=maximum pressure in the brakes, P_(TMC)=pressure ofthe actuating unit. A zero point adjustment is required because thecorrelation between the pressure difference at the separating valve 6and the current at which the separating valve 6 starts to open at thispressure difference is stored in the controller. Once open, theseparating valve 6 will close due to hysteresis effects, such asfriction, only when a lower pressure difference prevails again. Tosafely close an open separating valve 6 again, it must be energizedcorresponding to a pressure difference that is higher by a pressurevalue, e.g. 30 bar approximately.

Advantage: The separating valve 6 exhibits only low leakages. There areno closing noises.

Depending on the situation, the optimally suitable of the two strategiesis chosen per brake circuit. Strategy A is used with greatly differentvolume characteristic curves, for example

-   -   Systems with black-and-white circuit allotment    -   With ESP—OHB (optimized hydraulic brake) superposition    -   With ABS—OHB superposition

In this arrangement, the maximum wheel pressure can be predefined, withthe pump running.

Otherwise, strategy B is used to keep the separating valve 6 closed inthe best possible manner.

II. Maintaining the Pressure Constant

Possibility A:

The valve current I used to adjust the separating valve 6 is modifiedcorresponding to the characteristic curve I=I (P_(nominal)) illustratedin FIG. 3.

Advantage: The real pressure is limited in an upward direction. Excesspressure can be reduced. This may be necessary in the event ofsuperposition with other control functions of the device 28 such as ESP,ABS, etc.

Possibility B:

The valve current remains at I_(max).

Advantage: The valve is maximally seal-tight.

A pressure limitation of the hydraulic unit and the wheel brakes takesplace at a predefined pressure of the pressure limiting valve. PLVpressure P_(PLV) implies a pressure limitation that prevents amalfunction of the brake system or parts of the brake system. As thisoccurs, the separating valve 6 assumes a pressure limiting functionaccording to the relation I=I[min(P_(max)−P_(TMC), P_(PLV))], and theseparating valve 6 will open when the maximum pressure is exceeded, andpressure will discharge into the actuating unit 1. Below the maximumpressure, the separating valve 6 is being closed, or remains closed. Thecontrol unit 45 controls the separating valve 6 according to thedifferential pressures resulting in front of and behind the separatingvalve 6 or the pressure (behind the separating valve 6) in the hydraulicunit 43 and the wheel brakes 10, 11.

Possibility C:

When equal or rising differences in nominal pressure are detected for acertain time or number of loops in strategy A, the separating valve 6 isenergized corresponding to an additional pressure difference in order toclose it. With decreasing nominal pressure differences, there is aswitch-back into the status according to strategy A: energizationaccording to the nominal pressure. FIG. 4 depicts the variation of thenominal pressure P_(nominal) and the variation of the actual pressureP_(actual). The separating valve 6 is energized at time t₁ according toI=I (P_(nominal)+ΔP) in order to close it. The variation of P_(actual)without this current increase, which would result due to leakages, isreferred to by reference numeral 80.

III. Commencement of Pressure Reduction:

Possibility A:

Jump to the valve characteristic curve

Advantage: quick reaction

Possibility B:

Jump close to I=I (P_(nominal)+ΔP) of the valve characteristic curve.Thereafter follows a smooth, adapted transition by means of ramp 2 intoor onto the valve characteristic curve, i.e. I=I (P_(nominal))

Advantage: errors of the characteristic curve will no longer benoticeable.

Termination of boosting, the pressure of the actuating unit 1 is thewheel pressure again.

Current ramp according to I=0 (ramp 3).

Ramps: They are advantageous because errors in the characteristic curvesare not noticeable as an impact at the pedal but rather are removed byslow transitions that remain unnoticed. Further, noise development isgreatly reduced.

1-7. (canceled)
 8. Method for regulating a predetermined modifiablebrake pressure in the wheel brakes of a brake system, wherein inputquantities determining the brake pressure in the individual wheel brakesare evaluated and correcting variables of hydraulic valves are definedin a control and/or data processing system, wherein a characteristiccurve is stored in the control or data processing system, correlatingthe valve current of the hydraulic valve with a pressure difference ofthe hydraulic valve, and the correcting variable of the hydraulic valveis defined according to the characteristic curve.
 9. Method as claimedin claim 8, wherein the correcting variable is an electric valve currentused to control the hydraulic valve in analog manner.
 10. Method asclaimed in claim 8, wherein the correcting variable is determined fromthe pressure difference between the nominal pressure for the wheelbrakes and the pressure of an actuating device.
 11. Method as claimed inclaim 8, wherein at least two strategies are provided for regulating thebrake pressure, the valve current following the course of thecharacteristic curve according to the first strategy, while the secondstrategy arranges for jump functions at the maximum actuating current orthe actuating current represented by the characteristic curve. 12.Method as claimed in claim 11, wherein the maximum actuating current ofthe separating valve is limited according to the functionI_(max)=f(P_(max), P_(TMC), P_(PLV))=min(P_(max), P_(TMC), P_(PLV)). 13.Method as claimed in claim 12, wherein the separating valve is openedwhen the pressure in the hydraulic unit and the wheel brakes or adifferential pressure produced from the pressure in front of and behindthe separating valve exceeds a predefined value (P_(PLV)).
 14. Devicefor implementing the method as claimed in claim 8, wherein the variablebrake pressure generated by actuation of a pump is controlled in such away that a desired hydraulic boosting is achieved in each brake circuit.15. Method as claimed in claim 9, wherein the correcting variable isdetermined from the pressure difference between the nominal pressure forthe wheel brakes and the pressure of an actuating device.
 16. Device forimplementing the method as claimed in claim 9, wherein the variablebrake pressure generated by actuation of a pump is controlled in such away that a desired hydraulic boosting is achieved in each brake circuit.17. Device for implementing the method as claimed in claim 10, whereinthe variable brake pressure generated by actuation of a pump iscontrolled in such a way that a desired hydraulic boosting is achievedin each brake circuit.
 18. Device for implementing the method as claimedin claim 11, wherein the variable brake pressure generated by actuationof a pump is controlled in such a way that a desired hydraulic boostingis achieved in each brake circuit.
 19. Device for implementing themethod as claimed in claim 12, wherein the variable brake pressuregenerated by actuation of a pump is controlled in such a way that adesired hydraulic boosting is achieved in each brake circuit.